I use the one (Clock Gadget) packaged with Win7
Configure with Options button on gadget.http://windows.microsoft.com/en-us/windows-vista/Windows-Sidebar-and-gadgets-overview
Here are the instructions to ADD a second clock at desired time zone OR UTC.http://windows7themes.net/windows-7-world-clock-gadget.html
You don’t even need to download a world clock gadget in Windows 7 !
Why? Because there’s already a world clock built-in!
You can add up to two additional clocks to your taskbar (if you need more than two skip this part).